1. Field of the Invention
The present invention relates generally to the field of focusing of images for ultra high resolution synthetic aperture radar. Specifically, the invention provides a system and method for automatic estimation and correction of quadratic and cubic phase errors from range compressed synthetic aperture radar data to allow focus correction of the image.
2. Prior Art
High resolution synthetic aperture radar (SAR) is used for detailed ground mapping at long range. The data array for the SAR image which has been motion compensated to produce the focused image may still contain residual phase errors which result in an unfocused or smeared image. A number of effects may create this error, such as turbulence, errors in the velocity or inertial platform data from the aircraft, and variations in the height of ground features. However, the dominant source of error is an erroneously sensed motion of the aircraft caused by the inherent limitation of the inertial navigation system.
An automatic estimation and compensation of the phase error has classically been obtained using several techniques. The residual phase error is assumed representable in terms of a second or higher order polynomial and each auto focus technique attempts to estimate the coefficients of the assumed polynomial function. This phase error estimation usually involves partitioning the SAR array into several subarrays. One example is the phase comparison method found in U.S. Pat. No. 4,219,811 to Herman, et al. The Herman technique creates a vector resultant from each of three subarrays formed from the SAR array. A phase correction term is derived by comparing the phase angle of the first end subarray vector resultant with respect to the other end subarray vector resultant bisecting the angle of the two resultants and comparing it with the phase of the central subarray resultant.
Another example is the map drift auto focus method as discussed by C. E. Mancill and J. M. Swiger, published in the 27th Tri-service Radar Symposium Records, June 1981. In this method, multiple lower resolution images are produced from subarrays formed from the SAR array. The lower resolution images are then correlated to determine the relative shift. A set of relative shifts or "drifts" among images formed from subarrays are then processed to yield the coefficients of the assumed polynomial.
The phase comparison method is rarely used today, but it can perform a quadratic phase correction and has a pull-in range of approximately .+-.180 degrees. The map drift method has a very large pull-in range and is often used to estimate higher order phase errors in addition to a quadratic phase error. Focus correction provided by either method described will significantly improve the SAR image, provided that the underlying phase error can accurately be represented by a polynomial.
In the map drift autofocus method, images are formed from the subarrays and are pairwise-correlated to form correlation functions. The accurate determination of the underlying phase error is highly dependent on the presence of a clearly defined peak in the correlation function. The correlation function is, however, sensitive to the SAR scene being processed. If highly distinguishable targets such as man-made targets are present in the scene, the magnitude of the peak in the correlation function is pronounced and phase errors may be accurately estimated on that clearly defined peak. If, however, a SAR scene contains background clutter returns and man-made targets are absent, the correlation function is significantly flattened. As a result, the location of the peak cannot be accurately determined and the phase correction will be in error.
In addition, if the correlation function formed is of a biased type and the SAR scene does not contain man-made targets, a DC filter response can dominate all other correlation values, masking the "real" peak. If a false peak is detected, it will result in erroneous phase corrections.
Another shortcoming of the map drift autofocus method is its vulnerability to the presence of excessive residual phase errors. When subarrays are formed from a full array, a quadratic phase error is reduced quadratically. However, if the original full array has an extremely large amount of quadratic phase error, errors over subarrays may be still large enough to cause excessive defocusing in the resulting images, thus causing a peak of the correlation function to flatten. The map drift method must normally circumvent this shortcoming by performing an iterative focus.